Not the Same as Local Time

The term time zone is often confused with local time. For instance, during DST, it is common to say “California and Arizona are now in the same time zone.” However, the correct thing to say would be: “California and Arizona now have the same local time.”

Time Zone Borders Vary

Theoretically, each 1-hour time zone is 15 degrees wide, indicating a 1-hour difference in mean solar time. This can be seen as the white and gray stripes on our Time Zone Map and in the image above.

The actual borders on a time zone map have been drawn to correspond with both internal and international borders, and rarely match up exactly with the 15-degree time zone borders.

Some geographically large (wide) countries, like India and China, use only 1 time zone, while it would have been natural to expect several, like in the US or Australia.

Defined by UTC Offset

Every place on Earth is measured in terms of its distance east or west of the prime meridian (0°longitude) in Greenwich, London, United Kingdom. This is also the reference point for Coordinated Universal Time (UTC) with 1 hour per 15 degrees longitude.

You have to divide the longitude, in degrees, by 15 to find the appropriate time zone, in hours. For example:

At 150 degrees west (or 150° W) longitude, the time should be 150 degrees divided by 15 degrees = 10 hours behind UTC, or UTC-10.

At 75 degrees east (or 75° E) longitude, the time would be 75 degrees divided by 15 degrees = 5 hours ahead of UTC, or UTC+5.